1. Field of the Invention
This invention relates to method and apparatus for controlling tension in a web and more particularly to method and apparatus for maintaining a preselected tension on a continuous web of sheet material fed through a machine for making envelope blanks from the web.
2. Description of the Prior Art
In an envelope machine, envelopes are formed by cutting envelope blanks from a continuous roll of web material. Pull rolls pull the web under tension from a reel at a preselected feed rate. The web is fed to a cutter station where discrete lengths of envelope blanks are cut from the web. The length of the envelope blanks is determined by the ratio between the number of cuts per minute and the rate at which the web is fed to the cutter station.
The conventional practice is to vary the length of the blanks cut from the web within certain limits depending on the nature of the envelope to be formed from the envelope blank. Once the blanks are formed, they are then fed on the envelope machine to subsequent stations at preselected time intervals to perform a number of other given operations on the envelope blank. For example, at the front end of the machine, the envelope blanks must be in proper position for a rotating cutter knife or a panel cutter to cut windows or panels in the blanks. Thereafter, the blanks must be in proper position when the bottom seal score is impressed on the blank. Each operation requires that the blanks be of uniform length and are continuously fed at a preselected speed. Adjustments in the web tension are also required to be made over a period of time when the machine components are exposed to wear and adjustments must be made to maintain a desired tension in the web.
A conventional envelope machine includes a drive shaft that rotates at a preselected speed, and the web material is conveyed from a supply roll at a preselected rate feed relative to rotation of the drive shaft. Web cutting apparatus cuts the web material at preselected intervals to form various parts of the envelope blank, such as a bottom flap, a closure flap, side flaps, and a body portion of each envelope blank.
A drive mechanism is connected to a main drive shaft of the machine and includes a driven output shaft rotated relative to the rotation of the main drive shaft. The output shaft is, in turn, drivingly connected to the web feeding apparatus. The web feeding apparatus is then driven at a predetermined ratio relative to the main drive shaft. With this arrangement, the drive mechanism is operable to change the rate of rotation of the output shaft relative to the fixed rate of rotation of the drive shaft. This permits an adjustment to be made in the length of the envelope blank cut from the web and accordingly permits a change in the configuration of the envelope blank so that, for example, the length of the bottom flap can be changed while the closure flap and the body portion of the envelope are maintained a fixed length.
It has been the conventional practice to provide adjustments in the length of the envelope blanks cut from the web by connecting the drive shaft through a change gear unit to the web feeding apparatus. A gear set is used for the desired length of cut. Each gear set corresponds to a different feed rate and length of cut. While a variation in the feed length is provided, the length of cut is in increments. Substantially, infinitely variable feed lengths are not available with gear sets.
The change in feed length using gear sets in combination with a variable transmission necessitates an interruption in the operation of the machine to change the setting. Once the setting is changed, trial runs must be performed to determine if the setting change produces the desired length of envelope blank cut from the web. If the length of the envelope blank should deviate from the required length, then adjustments to the setting are required. Overall, the process of changing gear sets to change the length of the envelope blank is a time consuming operation. Furthermore, it necessitates the maintenance of a substantial inventory of gear sets to provide a full range of envelope blank sizes. U.S. Pat. Nos. 2,696,255; 3,056,322 and 3,128,662 are examples of envelope machines that utilize gear sets to provide adjustments in the length of envelope blanks cut from a web.
In an effort to increase the efficiency in changing the length of envelope blank cut from the web variable speed transmissions have been utilized to connect the main drive shaft with the web feeding apparatus. U.S. Pat. No. 4,020,722 discloses a cutting machine for cutting sheets from a web of paper in which a differential gear and a gear box drivingly connect the drive shaft to the web feeing apparatus. With this arrangement, the web feeding apparatus is driven at a preselected speed within a range without changing gear sets. The desired sheet length is set by setting the gear box at a ratio that drives the feeding apparatus for a preselected length of cut. Electrical pulses indicate of the speed at which the web is driven by the gear box are fed to a control unit and compared with the set sheet length. The comparison is computed and a resultant signal is transmitted to the pull rolls to correct the speed at which the web is fed to the cutter station.
U.S. Pat. No. 4,136,591 discloses in an envelope making machine apparatus for changing the length of envelope blanks cut from a continuous roll of web material in which a variable speed drive mechanism is connected to the drive shaft and includes an output shaft drivingly connected to web feeding apparatus. With this arrangement, the web feeding apparatus is driven at a predetermined ratio relative to the speed of the drive shaft. The variable speed mechanism is operable to change the speed of the output shaft relative to the speed of the input shaft to change the length of the bottom flap of an envelope blank while maintaining the closure flap and the body portion of the envelope a fixed length.
Other approaches to cutting envelope blanks of different lengths from a continuous web in envelope machines are disclosed in U.S. Pat. Nos. 1,837,727 and 3,056,322. U.S. Pat. No. 4,125,044 discloses in an envelope machine, a pair of feed rolls connected by a variable speed transmission to a drive motor. Cutting knives are positioned between the rollers. The rotational speed of the knives is adjusted relative to one another by the variable speed transmission.
U.S. Pat. No. 4,429,603 discloses in an envelope forming machine, a plurality of transmissions for obtaining desired speed ratios in adjusting the length of an envelope blank severed from the web. The relative gear ratios of the transmissions determine the length of the blank to be cut from the web and the length can be adjusted through the transmissions.
U.S. Pat. No. 3,244,045 discloses an input roller which feeds a strip of paper fed from a roll. The roller is drivingly connected through a gear train to a driven input shaft. A change gear in the gear train is mounted on an adjustable arm. The position of the arm is varied to accommodate different size change gears to vary the speed of the roller.
While it is known to provide adjustments in the length of the blank cut from a continuous web in an envelope making machine by change gears and by variable speed transmissions that transmit drive from the main drive shaft to the web feeding apparatus, the known devices are limited in the extent to which adjustments can be made to the feed length and web tension. Specific lengths are provided for specific gear sets. The variable speed transmission provides a degree of infinite adjustment within a range of size but not outside the range.
Therefore, there is need to provide in an envelope making machine apparatus that provides substantially infinite adjustment to the length of the envelope blank cut from the web while including an automatic control of the web tension. The machine must permit adjustments to be made in the feed length and web tension while the machine is running to avoid the necessity of shutting down operation of the machine to determine if the envelope blanks being cut correspond to the correct length or to prevent slack or excessive tension in the web.